Release No. 95-86
The Joint Damping Experiment (JDX) will provide information on how one type of joint used in space structures, a pin-joint, is able to dampen (dissipate) vibrations. Understanding how the absence of gravity changes the way components of a structure respond to vibrations is essential to the design of control systems that will actively control undesirable motions in future space structures, such as the International Space Station.
The pin-joint, which permits rotation about an axis, is frequently used to connect sections of deployable space structures (see NASA Langley photo: L-95-5001). Pin-joints generally dissipate vibrations faster than other joint types, such as hard mechanical joints, primarily because the components undergo very tiny movements relative to each other.
The proper testing of the JDX structure requires an environment free from external forces such as gravity, mechanical and acoustical vibrations, and air currents. With the exception of expendable rockets, the Space Shuttle is the only vehicle available that provides the necessary test conditions. Space testing is the third phase of this four-phase experiment. The first two involved extensive measurements in the laboratory and aboard an aircraft flying maneuvers to simulate the microgravity conditions of space. In phase four, the JDX/STS-69 flight data will be analysed and significant findings published.
The JDX test structure is constructed with struts and pin-joints to form a truss structure of three cubic bays (see JDX Test Structure figure). Each bay measures 8 x 8 x 8 inches, or approximately 20 x 20 x 20 centimeters. The truss structure is instrumented with precisely arranged, highly sensitive sensors to measure the magnitude and frequency of any movements. A unique arrangement of mechanical levers and electro-magnetic latches apply bending and twisting forces to the test structure. These movements are controlled by a battery powered controller/data acquisition system that automatically conducts the tests and stores the test data for post-mission analysis.
JDX will be flown in a Get-Away-Special (GAS) container in the payload bay of the Space Shuttle Endeavour. On orbit testing begins with one end of the JDX test structure pulled slightly to one side (the other end is fixed to the base of the GAS container) and then quickly released. The resulting motion is precisely measured as it decays with time. Next, the test structure is pulled in a direction 90 degrees to the first test, released, and the resulting motion recorded. In a third test, a force is applied to twist the test structure slightly. As before, the JDX test structure is released and the resulting motion measured as it decays with time. This sequence of three tests will be repeated 10 times during the mission.
Engineers will use the JDX/STS-69 flight data for three basic purposes: Provide structural engineers with specific values for pin-joint damping rates, assess the ability of ground-based and low-gravity aircraft flight tests to predict on-orbit test results and evaluate the ability of theoretical models to predict on-orbit damping rates of pinned-joints.
JDX is primarily a student-developed and executed space flight experiment at Utah State University in Ogden, Utah. Seven undergraduates and seven graduate students, as part of their engineering training, have been directly involved in all aspects of the project since it was selected by NASA in 1991. Four students were awarded master's degrees for their analysis of the JDX ground- and aircraft-based data and three others will receive master's degrees based on JDX/STS-69 flight data analyses.
JDX is funded by the NASA Headquarters' Office of Space Access and Technology (OSAT) through the In-Space Technology Experiments Program (IN-STEP). The total cost of the JDX program is $667,000 over the 4-1/2 year life of the project.